LTM4618 Datasheet, PDF (12/24 Pages) Linear Technology – 6A DC/DC μModule Regulator with Tracking and Frequency Synchronization

LTM4618
APPLICATIONS INFORMATION
The LTM4618 can be synchronized from 400kHz to 780kHz
with an input clock that has a high level above 2.0V and a
low level below 0.8V. The 400kHz low end operation limit
is put in place to limit inductor ripple current. See the
Typical Applications section for synchronization examples.
The LTM4618 minimum on-time is limited to about 90ns.
Guardband the on-time to 110ns. The on-time can be
calculated as:
t_ON(MIN)
=
1
FREQ
•
⎛
⎝⎜
V_OUT
V_IN
⎞
⎠⎟
Soft-Start and Tracking
LTM4618 has the ability to either soft-start by itself with a
capacitor or track the output of an external supply. When
the module is configured to soft-start by itself, a capacitor
should be connected to its TK/SS pin. When the module
is in the shutdown state, the TK/SS pin is actively pulled
to ground.
Once the RUN pin voltage is above 1.22V, the module pow-
ers up. Then a soft-start current of 1.3μA starts to charge
its soft-start capacitor. Note that soft-start or tracking is
achieved not by limiting the maximum output current of
the controller but by controlling the output ramp voltage
according to the ramp rate on the TK/SS pin. Current
foldback is disabled during this phase to ensure smooth
soft-start or tracking. The soft-start or tracking range is
defined as the voltage range from 0V to 0.8V on the TK/SS
pin. The total soft-start time can be calculated as:
t_SOFT-START
=
0.8V • C_SS
1.3µA
Output voltage tracking can be programmed externally
using the TK/SS pin. The master voltage is divided down
with an external resistor divider that is the same as the
slave’s feedback divider to implement coincident tracking.
The LTM4618 uses an accurate 60.4k resistor internally
for the top feedback resistor. Figure 4 shows an example
of coincident tracking.
V_OUT(SLAVE)
=
⎛
⎝⎜
1+
R1_⎞
R2 ^⎠⎟
•
V_TRACK
V_TRACK is the track ramp applied to the slave’s TK/SS
pin. V_TRACK has a control range of 0V to 0.8V. When the
master’s output is divided down with the same resistor
values used to set the slave’s output, then the slave will
coincident track with the master until it reaches its final
value. The master will continue to its final value from the
slave’s regulation point.
Ratiometric modes of tracking can be achieved by select-
ing different divider resistor values to change the output
tracking ratio. The master output must be greater than the
slave output for the tracking to work. Master and slave
data inputs can be used to implement the correct resistor
values for coincident or ratio tracking.
V_IN
5V
MASTER _R1
OUTPUT _60.4k
MODE/PLLIN INTV_CC EXTV_CC
V_IN
C_{IN COMP}
FREQ
V_OUT
LTM4618
TK/SS
V_FB
RUN
R2
28.7k
SGND
PGOOD
PGND
4618 F04
22pF
V_OUT(SLAVE)
2.5V/6A
C_OUT
28.7k
Figure 4. Output Voltage Coincident Tracking
OUTPUT
VOLTAGE
MASTER OUTPUT
SLAVE OUTPUT
TIME
4618 F05
Figure 5. Coincident Tracking Characteristics
4618f
12